Non-aqueous electrolyte secondary battery

ABSTRACT

A non-aqueous electrolyte secondary battery comprising: a positive electrode comprising a positive electrode current collector and a positive electrode material mixture layer formed thereon; a negative electrode comprising a negative electrode current collector and a negative electrode material mixture layer formed thereon; and a non-aqueous electrolyte, characterized in that at least one of said positive and negative electrodes has a positive temperature coefficient of resistance; and said non-aqueous electrolyte contains an additive which is stable in the normal operating voltage range of said battery and is able to polymerize at the voltage exceeding the maximum value of said operating voltage range.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte secondarybattery having a high level of safety.

BACKGROUND ART

More electronic devices have rapidly become portable and cordless thesedays, and the demand for smaller and lighter secondary batteries havinggreater energy density, which serve as power source to operate thesedevices, is increasing. Above all, great expectations are placed on anon-aqueous electrolyte secondary battery employing a negative electrodehaving lithium as an active material since it has high voltage and highenergy density.

The above-mentioned battery utilizes, for example, a lithium-containingmetal oxide for the positive electrode active material. Likewise,materials capable of absorbing and desorbing lithium, such as carbonmaterial, are employed for the negative electrode.

Assuring security is one of the important issues in non-aqueouselectrolyte secondary batteries. In particular, when a lithium ionsecondary battery is overcharged due to breakdown of a charge controlcircuit, excessive lithium ions in the positive electrode are extractedand migrate to the negative electrode. Accordingly, more of theprescribed amount of lithium is absorbed in the negative electrode or isdeposited on the negative electrode surface as metallic lithium. If itis forcibly kept charging in such a state, internal resistance in thebattery will increase, resulting in the excessive heat generation.

In order to cope with the excessive heat generation, it is proposed toprovide a positive temperature coefficient (PTC) thermistor or atemperature sensing type current breaking device such as fuse outsidethe battery. Use of the temperature sensing type current breaking deviceallows the electric current to be cut off without fail, thereby safetyof the battery can be ensured. Likewise, Japanese Laid-open patentpublication No. Hei 6-231749, Hei 10-125353 and Hei 10-241665 havesuggested a method to equip a current breaking device having a positivetemperature coefficient of resistance inside the battery. Thespecification of U.S. Pat. No. 4,943,497 further disclosed a means forcutting off the charge current by sensing a change in the internalpressure of the battery from a viewpoint of solving the problem ofovercharging. Referring conventional current breaking devices, however,cutting costs is difficult and providing them inside the small and thinbattery is structurally troublesome.

Consequently, Japanese Laid-open patent publication No. Hei 1-206571,Hei 6-338347 and Hei 7-302614 have suggested a method in which anadditive undergoing a reversible oxidation reduction reaction is addedto an electrolyte and electric energy fed into the inside of a batteryis self-consumed by redox shuttle mechanism. When the overcharge currentis increased, however, it can be hardly say that the battery employingthe redox shuttle mechanism is safe because the redox reaction rate andthe lithium ion moving rate have their limits.

At the same time, Japanese Laid-open patent publication No. Hei 9-50822,Hei 10-50342, Hei 9-106835, Hei 10-321258, Japanese Patent No. 2939469and Japanese Laid-open patent publication No. 2000-58117 have suggesteda means for adding aromatic compounds having a methoxy group and ahalogen element, biphenyl, thiophene, terphenyl, aromatic ether and thelike to the inside of a battery. These additives polymerize in themoderate overcharge process to cause temperature increase in thebattery. As a result, micropores of its separators are closed to cut offthe electric current.

In the battery having a temperature sensing type current breaking deviceoutside thereof and the one having a current breaking device having apositive temperature coefficient of resistance inside thereof asmentioned above, the device itself is heated and the resistance of thedevice is increased to cut off the current when a large amount ofcurrent, which is 5 to 6 times (5 to 6 C) or greater than the batterycapacity, flows during overcharge. Conversely, when they are overchargedat the normal charge and discharge current (1 to 2 C) of battery, safetycannot be fully ensured because there is not an adequate increase intemperature and resistance of the devices. However, use of a device inwhich resistance increases at a current of 1 to 2 C will impair batteryperformance.

When the battery having an electrolyte added with the aforementionedadditive is overcharged at the normal current (1 to 2 C), polymerizationof the additive on the electrodes and increase in electrode resistanceare observed. Conversely, when it is overcharged at a large current of 5to 6 C, safety cannot be fully ensured because the polymerization of theadditive lags behind the charge.

In the case that an additive is added to the electrolyte in the batteryhaving a temperature sensing type current breaking device outside thebattery, safety is ensured when overcharged at a current of 1 to 2 C ora large current of 5 C or more. When overcharged at a current of 3 to 5C, however, safety is not fully assured because the temperature sensingtype current breaking device does not operate sensitively and thepolymerization of the additive lags behind the charge.

In view of the aforementioned facts, an object of the present inventionis to provide a battery wherein safety is ensured even when it isovercharged in any current range.

DISCLOSURE OF INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery comprising: a positive electrode comprising a positive electrodecurrent collector and a positive electrode material mixture layer formedthereon; a negative electrode comprising a negative electrode currentcollector and a negative electrode material mixture layer formedthereon; and a non-aqueous electrolyte, characterized in that (1) atleast one of the positive and negative electrodes has a positivetemperature coefficient of resistance, (2) the non-aqueous electrolytecontains an additive which is stable at a normal operating voltage rangeof the battery and is able to polymerize at the voltage exceeding themaximum value of the above operating voltage range.

The non-aqueous electrolyte includes a liquid electrolyte comprising asolute and a non-aqueous solvent, a gel electrolyte comprising a hostpolymer retaining a liquid electrolyte, a solid polymer-electrolytecontaining a solute.

It is preferable that a resistance value at 110 to 130° C. of at leastone of the positive and negative electrodes is 100 times or greater thana resistance value at 25° C. of the same electrode. It is desirable, forinstance, that the resistance value suddenly increases at around 120° C.to a hundredfold or more of the resistance value at room temperature.

It is preferable that a resistivity at 120° C. of at least one of thepositive and negative electrodes is 10⁷ Ω·cm or more.

Any additive capable of polymerizing at the voltage exceeding the upperlimit value of the normal operating voltage range of a battery may beutilized without limitations. In particular, it is effective to use atleast one selected from a group consisting of biphenyl,3-chlorothiophene, furan, o-terphenyl, m-terphenyl, p-terphenyl,diphenyl ether, 2,3-benzofuran, bis(p-tolyl)ether, diallyl ether, allylbutyl ether, 3-phenoxy toluene and cyclohexyl benzene. These additivesmay be employed alone or in combination of two or more.

It is effective to form a resistance layer having a positive temperaturecoefficient of resistance on the surface of the positive or negativeelectrode current collector in order to impart a positive temperaturecoefficient of resistance to the positive or negative electrode.

In the case of the positive electrode, for instance, it is effective toprovide a resistance layer comprising a mixture of a conductiveparticulate material and a binder polymer on the surface of a currentcollector comprising aluminum. It is desirable that the conductiveparticulate material for the use of the positive electrode comprises acarbon material.

In the case of the negative electrode, for example, it is effective toprovide a resistance layer comprising a mixture of a conductiveparticulate material and a binder polymer on the surface of a negativeelectrode current collector comprising copper or nickel. It is desirablethat the conductive particulate material for the use of the negativeelectrode comprises nickel or copper.

It is desirable that a binder polymer for the positive or negativeelectrode is at least one selected from the group consisting ofpolyethylene, ethylene-vinyl acetate copolymer, ethylene-propylenecopolymer, ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile and aromatic hydrocarbon having a vinylgroup-conjugated diene copolymer in that the strength of the electrodeplate and the temperature coefficient of resistance are easilycontrolled. These binder polymers may be used alone or in combination oftwo or more.

It is desirable that a ratio of battery capacity C (mAh) to facing areaA (cm²) of the positive and negative electrodes: C/A value is 0.2 to 6.0mAh/cm². In particular, it is preferable that C/A value is 0.2 to 4.5mAh/cm² in that the high rate characteristics of the battery can besufficiently maintained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical section of a cylindrical battery in accordance withan example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the non-aqueous electrolyte secondary battery of the presentinvention, (1) at least one of the positive and negative electrodes hasa positive temperature coefficient of resistance, (2) the non-aqueouselectrolyte contains an additive which is stable at a normal operatingvoltage range of the battery and is able to polymerize at the voltageexceeding the maximum value of the operating voltage range.

The above-mentioned additive starts to polymerize on the positiveelectrode when the battery becomes overcharged. Since a copolymer isproduced on the surface of the positive electrode, theoxidation-reduction reaction involving active material is impaired, theinternal resistance of the battery starts to increase, thereby thebattery heats. At this time, the temperature of the electrode platehaving a positive temperature coefficient of resistance increasessimultaneously, and the resistance of the electrode plate rises.Synergistic interaction between the increase in the internal resistancedue to the additive and the rise in the electrode plate resistanceallows the current to be promptly cut off even when the battery isovercharged at a current of 3 to 5 C.

Additionally, the polymerization reaction on the positive electroderemarkably decreases the efficiency in extraction of lithium ions fromthe positive electrode; therefore, decrease in the thermal stability ofthe positive electrode active material can also be prevented. Therefore,the present invention can practically provide extremely safer batteries,compared to the conventional ones.

In general, s temperature coefficient of resistance X can be expressedas:X(ppm/° C.)=(R−R ₀)/R ₀(t−t ₀)×10⁶.

where R represents resistance (Ω) at t° C., R₀ represents resistance (Ω)at t₀° C.

The temperature coefficient of resistance X of at least one of thepositive and negative electrodes for the use of the present invention ispreferably 1×10⁶≦X≦1×10¹⁰. It should be noted that the temperaturecoefficient of resistance X of the same electrode preferably undergoes agreat change at, for example, 110 to 130° C.

The additive to be added to the electrolyte is not intended for theredox shuttle mechanism as described in Japanese Laid-Open PatentPublication No. Hei 7-302614 and Hei 9-50822. Accordingly, it isdesirable that the oxidative polymerization of the additive isirreversible.

The aforementioned additive is required to be chemically stable at anormal operating voltage range of the battery, and immediate oxidativepolymerization is necessary at the voltage in the overcharge rangeexceeding the maximum value of the operating voltage range. When alithium containing transition metal oxide such as LiCoO₂, LiNiO₂ orLiMn₂O₄ is used for the positive electrode active material and a carbonmaterial is employed for the negative electrode, for instance, theadditive is stable at 0.03 to 4.3 V, but immediate oxidativepolymerization is necessary at over 4.3 V.

It is preferable to employ, for example, biphenyl, 3-chlorothiophene,furan, o-terphenyl, m-terphenyl, p-terphenyl, diphenyl ether,2,3-benzofuran, bis(p-tolyl)ether, diallyl ether, allyl butyl ether,3-phenoxy toluene or cyclohexyl benzene for the above-mentionedadditive. These may be used alone or in combination of two or more.These do not affect the battery performance as long as they are utilizedin the normal voltage range, and they act effectively when the batteryis overcharged.

It is effective to add 0.5 to 5 parts by weight of additive per 100parts by weight of non-aqueous electrolyte.

The positive and negative electrodes usually have a plate-like shape. Itis preferable that a ratio of battery capacity C (mAh) to facing area A(cm²) of the positive and negative electrodes: C/A value is 0.2 to 6.0mAh/cm². When C/A value is less than 0.2 mAh/cm², in other words, whenthe electrode plate area is too large against capacity C, it is notpractical from the viewpoint of battery capacity. Conversely, when C/Avalue is over 6.0 mAh/cm², if the battery is charged and discharged withthe normal current, the current density increases to provide a highpossibility of raising the resistance of the electrode plate, therefore,it is not practical, either. Likewise, from the viewpoint of the highrate discharge characteristics of the battery, it is further preferablethat C/A value is not more than 4.5 mAh/cm².

Any electronic conductor that does not induce chemical reaction insidethe battery may be employed for the positive electrode currentcollector. The positive electrode current collector can be composed of,for example, stainless steel, aluminum, titanium or carbon. Among them,aluminum and aluminum alloy are particularly preferable.

As the positive electrode current collector, there are a foil, a film, asheet, a net, a punched sheet, a lath, a porous sheet, a foam and amolded article formed by fiber bundle or non-woven fabric. The surfaceof the positive electrode current collector may be made rough by asurface treatment. The positive electrode current collector has athickness of, for example, 1 to 500 μm.

Formation of a resistance layer having a positive temperaturecoefficient of resistance on the surface of the positive electrodecurrent collector can give a positive electrode having a positivetemperature coefficient of resistance.

Desirably, the resistance layer for the positive electrode comprises amixture of a conductive particulate material and a binder polymer. Theresistance layer is provided such that the surface of the positiveelectrode current collector is coated with the above-mentioned mixtureto a thickness of, for example, 0.5 to 10 μm.

The conductive particulate material for the positive electrode desirablycomprises a carbon material such as acetylene black or artificialgraphite.

As the binder polymer for the positive electrode, polyethylene,ethylene-vinyl acetate copolymer, ethylene-propylene copolymer,ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile, aromatic hydrocarbon having a vinyl group-conjugateddiene copolymer (styrene-butadiene copolymer, for instance) or the likeis preferable. These expand in high temperatures, which decreases thecontact point of the particulate material and raises the resistance ofthe resistance layer. The binder polymer having a softening temperatureof 110 to 130° C. is particularly effective.

In the resistance layer for the positive electrode, it is preferable tomix 0.5 to 10 parts by weight of, further preferably 0.5 to 5 parts byweight of, binder polymer with 100 parts by weight of particulatematerial.

As the positive electrode active material contained in the positiveelectrode material mixture layer, lithium-containing composite oxide ispreferable. The lithium-containing composite oxide includesLi_(x)CoO_(z), Li_(x)NiO_(z), Li_(x)MnO_(z), Li_(x)Co_(y)Ni_(1−y)O_(z),Li_(x)Co_(f)V_(1−f)O_(z), Li_(x)Ni_(1−y)M_(y)O_(z) (M=Ti, V, Mn, Fe),Li_(x)Co_(a)Ni_(b)M_(c)O_(z) (M=Ti, Mn, Al, Mg, Fe, Zr), Li_(x)Mn₂O₄,Li_(x)Mn_(2(1−y))M_(2y)O₄ (M=Na, Mg, Sc, Y, Fe, Co, Ni, Ti, Zr, Cu, Zn,Al, Pb, Sb) (where x=0 to 1.2, y=0 to 1.0, f=0.9 to 0.98, z=1.9 to 2.3,a+b+c=1.0, 0≦a≦1, 0≦b≦1, 0≦c<1). The x value mentioned above is the onebefore the start of charge and discharge of the battery; therefore, itvaries with the charge and discharge. A plurality of different positiveelectrode active materials may be used simultaneously.

Lithium-containing composite oxide is synthesized by mixing carbonate,nitrate, oxide or hydroxide of lithium with carbonate, nitrate, oxide orhydroxide of a transition metal at a desired composition and pulverizingthem, and then calcining the mixture. The calcination temperature is 250to 1500° C., at which a portion of the materials is decomposed ormolten. The calcination time is preferably 1 to 80 hours.

Any electronic conductor that does not induce chemical reaction insidethe battery may be employed for the negative electrode currentcollector. The negative electrode current collector can be composed of,for example, stainless steel, nickel, copper or titanium. Among them,copper or copper alloy is preferable.

As the negative electrode current collector, there are a foil, a film, asheet, a net, a punched sheet, a lath, a porous sheet, a foam and amolded article formed by fiber bundle or non-woven fabric. The surfaceof the negative electrode current collector may be made rough by asurface treatment. The negative electrode current collector has athickness of, for instance, 1 to 500 μm, and preferably, 1 to 15 μm.

Formation of a resistance layer having a positive temperaturecoefficient of resistance on the surface of the negative electrodecurrent collector can give a negative electrode having a positivetemperature coefficient of resistance.

Desirably, the resistance layer for the negative electrode comprises amixture of a conductive particulate material and a binder polymer. Theresistance layer is provided such that the surface of the negativeelectrode current collector is coated with the above-mentioned mixtureto a thickness of, for example, 0.5 to 10 μm.

The conductive particulate material for the negative electrode desirablycomprises a chemically stable material such as nickel or copper. It isdesirable that the mean diameter of the particulate material is 0.5 to10 μm.

The binder polymer for the negative electrode preferably comprisespolyethylene, ethylene-vinyl acetate copolymer, ethylene-propylenecopolymer, ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile, aromatic hydrocarbon having a vinyl group-conjugateddiene copolymer or the like.

In the resistance layer for the negative electrode, it is preferable tomix 0.5 to 10 parts by weight, further preferably 0.5 to 5 parts byweight, of binder polymer with 100 parts by weight of particulatematerial.

As the material contained in the negative electrode material mixturelayer, there are lithium alloy, alloy, intermetallic compound, carbonmaterial, organic compound, inorganic compound, metal complex andorganic polymer compound. These may be used alone or in combination.

As the carbon material, there are coke, thermally decomposed carbon,natural graphite, artificial graphite, mesocarbone microbeads,graphitized mesophase spherules, vapor phase growth carbon, glassycarbon, carbon fiber, amorphous carbon, and calcined product of organiccompound. These may be employed alone or in combination. Among them,graphitized mesophase spherules, natural graphite, or artificialgraphite is preferable.

When Li is contained in the positive electrode active material, carboncontaining no Li can be employed for the negative electrode material.Incidentally, it is preferable that 0.01 to 10 parts by weight of Li iscontained per 100 parts by weight of the negative electrode materialcontaining no Li originally. In order to allow the negative electrodematerial to contain Li, it is recommendable that metallic lithium havingbeen heated and molten is applied onto the current collector with thenegative electrode material being pressed thereto, or that the negativeelectrode material is electrochemically doped with lithium inelectrolyte after the metallic lithium is attached to the negativeelectrode.

As the binder contained in the positive or negative electrode materialmixture, there are fluorocarbon resin such as polyvinylidene fluoride orpolytetrafluoroethylene, acrylic resin, styrene-butadiene rubber andethylene-propylene copolymer. These may be used alone or in combination.

The non-aqueous electrolyte employed in the present invention preferablycomprises a non-aqueous solvent and a lithium salt. As the non-aqueoussolvent, for instance, cyclic carbonate such as ethylene carbonate,propylene carbonate, butylene carbonate or vinylene carbonate; noncycliccarbonate such as dimethyl carbonate, diethyl carbonate, ethyl-methylcarbonate, methyl propyl carbonate, methyl-isopropyl carbonate ordipropyl carbonate; aliphatic carboxylic acid ester such as methylformate, methyl acetate, methyl propionate or ethyl propionate;γ-lactone such as γ-butyrolactone; noncyclic ether such as1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxy methoxy ethane; cyclicether such as tetrahydrofuran, 2-methyl tetrahydrofuran; alkyl phosphateor its fluoride such as dimethyl sulfoxide, 1,3-dioxolane, trimethylphosphate, triethyl phosphate or trioctyl phosphate. These may beemployed alone or in combination of two or more. Among them, it ispreferable to use a mixture of cyclic carbonate and noncyclic carbonate,or a mixture of cyclic carbonate, noncyclic carbonate and aliphaticcarboxylic acid ester.

As the lithium salt to be dissolved in the non-aqueous solvent, LiClO₄,LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆, LiSCN, LiCl, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆,LiN(CF₃SO₂)₂, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃ andthe like can be cited as examples. These may be used alone or incombination of two or more. It is particularly preferable to employLiPF₆. As a particularly preferable non-aqueous electrolyte, there is anon-aqueous electrolyte comprising LiPF₆ dissolved in a mixed solventcomprising ethylene carbonate and ethylmethyl carbonate.

A preferable concentration of the lithium salt in the non-aqueouselectrolyte is 0.2 to 2 mol/liter, and 0.5 to 1.5 mol/liter ispreferable in particular. The amount of non-aqueous electrolyte to beinjected into the battery varies according to the capacity of electrodesand the size of the battery.

A gel electrolyte wherein a liquid non-aqueous electrolyte having beenmade retained in a host polymer can be employed. As the host polymer,there are polyethylene oxide, polypropylene oxide, polyvinylidenefluoride, and their derivatives, for instance. Especially, a copolymerof polyvinylidene fluoride and hexafluoropropylene or a mixture ofpolyvinylidene fluoride and polyethylene oxide is preferable.

As the separator, an insulative microporous thin film or a non-wovenfabric having a great ion permeability and a desired mechanicalstrength, is utilized. It is preferable to use a separator having afunction to close its micropores at a temperature of 80° C. or higher.The separator is composed of olefin such as polypropylene, polyethylene,glass fiber or the like from the viewpoint of resistance to thenon-aqueous solvent and hydrophobicity. A preferable micropore diameterof the separator is the size that the active material, binder orconductive agent come off the electrode plate cannot pass through,specifically, 0.01 to 1 μm. Generally, the separator has a thickness of5 to 300 μm and a porosity of 30 to 80%.

The present invention can be applied to any form of battery such assheet, cylindrical, flat or square. In the case of cylindrical or squarebattery, cylindrical electrode assembly or the one having an ellipsesection is constituted such that the positive and negative electrodessandwiching a separator therebetween are laminated and wound. It ispreferable to provide a safety valve in the battery.

Examples of the present invention are described below referring to thedrawing.

EXAMPLE 1

(i) Positive Electrode

Acetylene black serving as the conductive particulate material andpolyethylene serving as the binder polymer and having a softeningtemperature of 120° C. were mixed in a weight ratio of 10:1, and aproper amount of carboxymethyl cellulose was added thereto as thethickener to give a paste like mixture.

Both sides of a 10 μm thick aluminum foil serving as the positiveelectrode current collector were coated with the aforementioned mixtureto a thickness of 5 μm or less, and dried to yield a resistance layer.

The positive electrode material mixture was prepared by mixing 100 partsby weight of LiCoO₂ powder, 3 parts by weight of acetylene black and 7parts by weight of fluorocarbon resin type binder(polytetrafluoroethylene) with a proper amount of carboxymethylcellulose aqueous solution. The above-mentioned LiCoO₂ powder wassynthesized by calcining a mixture of Li₂CO₃ and Co₃O₄ at 900° C. for 10hours.

Both sides of the positive electrode current collector having aresistance layer were coated with the obtained positive electrodematerial mixture, dried and rolled with pressure to give a positiveelectrode having a thickness of 0.17 mm, a width of 55 mm and a lengthof 540 mm. A lead made of aluminum was attached to the positiveelectrode.

The resistance value of the obtained positive electrode having theresistance layer was observed to suddenly increase at around 120° C.,which is the softening temperature of polyethylene, to a hundredfold ormore of the resistance value at room temperature. The resistance valueof the positive electrode at 120° C. was 3.0×10⁷ Ω·cm.

(ii) Negative Electrode

The negative electrode material mixture was prepared by mixing 100 partsby weight of mesophase graphite and 5 parts by weight ofstyrene-butadiene rubber with carboxymethyl cellulose aqueous solution.The aforementioned mesophase graphite was prepared by graphitizingmesophase microspheres at 2800° C.

Both sides of 0.01 mm thick Cu foil were coated with the aforementionednegative electrode material mixture, dried and rolled with pressure toproduce a negative electrode having a thickness of 0.156 mm, a width of56 mm and a length of 585 mm. A lead made of nickel was attached to thenegative electrode.

(iii) Non-Aqueous Electrolyte

The non-aqueous electrolyte was prepared by dissolving LiPF₆ in a mixedsolvent comprising ethylene carbonate and diethyl carbonate at a volumeratio of 30:70, at a concentration of 1 mol/liter. 2 parts by weight ofbiphenyl per 100 parts by weight of the non-aqueous electrolyte wasadded as the additive.

(iv) The Assembly of a Cylindrical Battery

It is described referring to FIG. 1.

A battery case 1 having a diameter of 18.0 mm and a height of 65.0 mmwas produced by processing a stainless steel plate. The battery case 1housed an electrode assembly 4 wherein a positive electrode 5 and anegative electrode 6 sandwiching a polyethylene separator 7 having athickness of 0.018 mm, a width of 58 mm and a length of 1430 mm werespirally wound. Insulating rings 8 were respectively provided on andbeneath the electrode assembly 4. The positive electrode 5 and a sealingplate 2 were connected with a positive electrode lead 5 a, and thenegative electrode 6 and the bottom surface inside the battery case 1were connected with a negative electrode lead 6 a. The opening of thebattery case 1 was sealed with the sealing plate 2 having a safety valveand an insulating packing 3 after the above-mentioned non-aqueouselectrolyte was poured therefrom, thereby a battery 1 was obtained. TheBattery 1 had a capacity C of 2100 mAh, and a ratio of battery capacityC to facing area A of the positive and negative electrodes: C/A was 3.54mAh/cm².

EXAMPLE 2

Cylindrical batteries were constructed as in Example 1 except that3-chlorothiophene, furan, o-terphenyl, m-terphenyl, p-terphenyl,diphenyl ether, 2,3-benzofuran, bis(p-tolyl)ether, diallyl ether, allylbutyl ether, 3-phenoxy toluene or cyclohexyl benzene was used as theadditive. They were named as Batteries 2 to 13. C/A value of Batteries 2to 13 was also 3.54 mAh/cm², which was the same as that of Battery 1.

EXAMPLE 3

(i) Both sides of 10 μm thick aluminum foil serving as the positiveelectrode current collector were directly coated with the same positiveelectrode material mixture as that in Example 1, dried and rolled withpressure to give a positive electrode having a thickness of 0.16 mm, awidth of 55 mm and a length of 540 mm. Accordingly, this electrode doesnot have a resistance layer.

(ii) Nickel powder serving as the conductive particulate material andpolyethylene serving as the binder polymer and having a softeningtemperature of 120° C. were mixed in a weight ratio of 10:1, and aproper amount of carboxymethyl cellulose was added thereto as thethickener to give a paste like mixture.

Both sides of 10 μm thick copper foil serving as the negative electrodecurrent collector were coated with the aforementioned mixture to athickness of 5 μm or less, and dried to provide a resistance layer.

Both sides of the negative electrode current collector having theobtained resistance layer were coated with the same negative electrodematerial mixture as that in Example 1, dried and rolled with pressure toproduce a negative electrode having a thickness of 0.166 mm, a width of56 mm and a length of 585 mm.

The resistance value of the obtained negative electrode having theresistance layer was observed to suddenly increase at around 120° C.,which is the softening temperature of the polyethylene, to a hundredfoldor more of the resistance value at room temperature.

Battery 14 was produced as Battery 1 of Example 1 with the use of theobtained positive and negative electrodes. The battery 14 had a capacityC of 2100 mAh, and a ratio of battery capacity C to facing area A of thepositive and negative electrodes: C/A was 3.54 mAh/cm².

EXAMPLE 4

The length of the positive electrode current collector having theresistance layer and the amount of the positive electrode materialmixture to be applied thereon were adjusted to obtain a positiveelectrode having a thickness of 0.0183 mm, a width of 55 mm and a lengthof 2800 mm. Likewise, the length of the negative electrode currentcollector and the amount of the negative electrode material mixture tobe applied thereon were adjusted to obtain a negative electrode having athickness of 0.017 mm, a width of 56 mm and a length of 2845 mm. Battery15 was constructed as in Example 1 except that the above-mentionedpositive and negative electrodes and a polyethylene separator having athickness of 0.018 mm, a width of 58 mm and a length of 5950 mm wereused. Battery 15 had a capacity C of 632 mAh, and a ratio of batterycapacity C to facing area A of the positive and negative electrodes: C/Awas 0.205 mAh/cm².

EXAMPLE 5

The length of the positive electrode current collector having theresistance layer and the amount of the positive electrode materialmixture to be applied thereon were adjusted to obtain a positiveelectrode having a thickness of 0.284 mm, a width of 55 mm and a lengthof 330 mm. Likewise, the length of the negative electrode currentcollector and the amount of the negative electrode material mixture tobe applied thereon were adjusted to obtain a negative electrode having athickness of 0.26 mm, a width of 56 mm and a length of 375 mm. Battery16 was constructed as in Example 1 except that the above-mentionedpositive and negative electrodes and a polyethylene separator having athickness of 0.018 mm, a width of 58 mm and a length of 1010 mm wereused. Battery 16 had a capacity C of 2180 mAh, and a ratio of batterycapacity C to facing area A of the positive and negative electrodes: C/Awas 6.01 mAh/cm².

EXAMPLE 6

The length of the positive electrode current collector having theresistance layer and the amount of the positive electrode materialmixture to be applied thereon were adjusted to obtain a positiveelectrode having a thickness of 0.214 mm, a width of 55 mm and a lengthof 440 mm. Likewise, the length of the negative electrode currentcollector and the amount of the negative electrode material mixture tobe applied thereon were adjusted to obtain a negative electrode having athickness of 0.20 mm, a width of 56 mm and a length of 485 mm. Battery17 was constructed as in Example 1 except that the above-mentionedpositive and negative electrodes and a polyethylene separator having athickness of 0.18 mm, a width of 58 mm and a length of 1230 mm wereused. Battery 17 had a capacity C of 2140 mAh, and a ratio of batterycapacity C to facing area A of the positive and negative electrodes: C/Awas 4.5 mAh/cm².

COMPARATIVE EXAMPLE 1

Battery 18 which is the same as that of Example 1 was constructed exceptthat no resistance layer was provided on the positive electrode currentcollector and no additive was added to the non-aqueous electrolyte.

COMPARATIVE EXAMPLE 2

Battery 19 which is the same as that of Example 1 was constructed exceptthat no resistance layer was provided on the positive electrode currentcollector. Accordingly, Battery 19 does not have a resistance layer onthe positive electrode, but its non-aqueous electrolyte contains theadditive.

COMPARATIVE EXAMPLE 3

Battery 20 which is the same as that of Example 1 was constructed exceptthat no additive was added to the electrolyte. Accordingly, Battery 20has a resistance layer on the positive electrode, but its non-aqueouselectrolyte does not contain the additive.

COMPARATIVE EXAMPLE 4

Battery 21 which is the same as battery 19 of Comparative Example 2 wasconstructed except that a PTC device was serially arranged on the outersurface of the battery case.

EXAMPLE 7

The length of the positive electrode current collector having theresistance layer and the amount of the positive electrode materialmixture to be applied thereon were adjusted to obtain a positiveelectrode having a thickness of 0.34 mm, a width of 55 mm and a lengthof 275 mm. Likewise, the length of the negative electrode currentcollector and the amount of the negative electrode material mixture tobe applied thereon were adjusted to obtain a negative electrode having athickness of 0.31 mm, a width of 56 mm and a length of 320 mm. Battery22 which is the same as that of Example 1 was constructed except thatthe above-mentioned positive and negative electrodes and a polyethyleneseparator having a thickness of 0.018 mm, a width of 58 mm and a lengthof 900 mm were used. Battery 22 had a capacity C of 2180 mAh, and aratio of battery capacity C to facing area A of the positive andnegative electrodes: C/A was 7.2 mAh/cm².

EXAMPLE 8

The length of the positive electrode current collector having theresistance layer and the amount of the positive electrode materialmixture to be applied thereon were adjusted to obtain a positiveelectrode having a thickness of 0.0158 mm, a width of 55 mm and a lengthof 3040 mm. Likewise, the length of the negative electrode currentcollector and the amount of the negative electrode material mixture tobe applied thereon were adjusted to obtain a negative electrode having athickness of 0.015 mm, a width of 56 mm and a length of 3085 mm. Battery23 which is the same as that of Example 1 was constructed except thatthe above-mentioned positive and negative electrodes and a polyethyleneseparator having a thickness of 0.018 mm, a width of 58 mm and a lengthof 6430 mm were used. Battery 23 had a capacity C of 502 mAh, and aratio of battery capacity C to facing area A of the positive andnegative electrodes: C/A was 0.15 mAh/cm².

Evaluation of Batteries

Overcharge was conducted on Batteries 1 to 23 by using 20 cells each.

Fully charged batteries at an environmental temperature of 20° C. werefurther overcharged at a current of 1 C, 3 C or 6 C to see whether thebatteries would cause excessive heat generation. Tables 1 and 2 show thenumber of cells where excessive heat generation occurred.

TABLE 1 No. Additives 1 C 3 C 6 C 1 Biphenyl 0/20 0/20 0/20 23-chlorothiophene 0/20 0/20 0/20 3 Furan 0/20 0/20 0/20 4 o-terphenyl0/20 0/20 0/20 5 m-terphenyl 0/20 0/20 0/20 6 p-terphenyl 0/20 0/20 0/207 Diphenyl ether 0/20 0/20 0/20 8 2,3-benzofuran 0/20 0/20 0/20 9Bis(p-tolyl)ether 0/20 0/20 0/20 10 Diallyl ether 0/20 0/20 0/20 11Allyl butyl ether 0/20 0/20 0/20 12 3-phenoxy toluene 0/20 0/20 0/20 13Cyclohexyl benzene 0/20 0/20 0/20 14 Biphenyl 0/20 0/20 0/20 15 Biphenyl0/20 0/20 0/20 16 Biphenyl 0/20 0/20 0/20 17 Biphenyl 0/20 0/20 0/20

TABLE 2 No. Additives 1 C 3 C 6 C 18 None 18/20 20/20 20/20 19 Biphenyl 0/20  5/20 12/20 20 None 15/20 12/20  0/20 21 Biphenyl  0/20  7/20 0/20 22 Biphenyl  0/20  0/20  0/20 23 Biphenyl  0/20  0/20  0/20

As shown in Table 2, in Battery 18 of Comparative Example 1 withoutaddition of an additive to the non-aqueous electrolyte and without anelectrode plate having a positive temperature coefficient of resistance,excessive heat generation was observed in almost all of 20 cells when itwas overcharged with whichever electric current.

In Battery 19 of Comparative Example 2 with addition of an additive tothe non-aqueous electrolyte, excessive heat generation was able to beprevented at the normal charge current of 1 C, but the probability ofexcessive heat generation became high as the current value was increasedto 3 C and 6 C. This is considered to be because increase in the chargecurrent value decreases current efficiency in the polymerizationreaction of the additive, and a lot of lithium is extracted from thepositive electrode, as a result, safety is lowered. Accordingly, inorder to improve safety during overcharge, it is inadequate just to addan additive.

In Battery 20 of Comparative Example 3 having the electrode plate havinga positive temperature coefficient of resistance, excessive heatgeneration was able to be prevented when it was overcharged with a largecurrent such as 6 C. This is considered to be because the binder polymerof the resistance layer expanded due to self heating to cut off theelectronic network of the conductive particles, as a result, internalresistance increases rapidly and the electric current does not flow tothe inside of the battery. When it was overcharged with a low currentvalue of 1 C or 3 C, however, the resistance value did not rise,therefore, it is presumed that overcharge is allowed to proceed and alot of lithium is extracted from the positive electrode, leading tolower safety.

When Battery 21 of Comparative Example 4, in which an additive was addedto the non-aqueous electrolyte and a PTC was located outside thebattery, was overcharged at a low current value of 1 C and at a highcurrent value of 6 C, the additive and the PTC respectively functionedwell, and safety was ensured; however, when it was overcharged at acurrent value of 3 C, the PTC did not work, and in addition to that, thecurrent efficiency of polymerization reaction of the additive was alsolow, therefore, the ratio of battery causing excessive heat generationwas 7/20, which was quite large.

On the other hand, Batteries 1 to 13 of the present invention did notcause excessive heat generation at all at every current value of 1 to 6C. This is considered to be because the additive is polymerized to forma film on the surface of the positive electrode, and the reactionresistance increases to generate heat, resulting in rapid increase inresistance of the resistance layer provided on the electrode plate.Battery 14 of Example 3 employing the negative electrode having apositive temperature coefficient of resistance also showed similareffect.

The trip temperature where the charge efficiency of polymerizationreaction of the additive and the resistance of the electrode platehaving a positive temperature coefficient of resistance change suddenlyis closely related to the current density during charging.

It has been ascertained from the results of Battery 15 of Example 4,Battery 16 of Example 5 and Battery 17 of Example 6 that an effect ofthe present invention, which is to improve safety, can be similarlyensured in the range that a ratio of battery capacity C to facing area Aof the positive and negative electrodes: C/A is 0.2 to 6.0 mAh/cm².

On the other hand, the ratio of the discharge capacity at 2 Cdischarging to the discharge capacity at 0.2 C discharging differedconsiderably as 25% for Battery 16 having a C/A value of 6.0 mAh/cm² and70% for Battery 17 having a C/A value of 4.5 mAh/cm². Accordingly, itcan be said that C/A value is preferably 4.5 mAh/cm² or less from theviewpoint of high rate discharge characteristics.

Battery 22 of Example 7 having a C/A value of 7.2 mAh/cm² had a highlevel of safety, but it had large current densities even in the normalcurrent range of charge and discharge; therefore, resistance of theresistance layer sometimes increased during 2 C discharging.Consequently, it can be said that C/A is preferably 6.0 mAh/cm² or less.

Likewise, in Battery 23 of Example 8 having a C/A value of 0.15 mAh/cm²,safety was secured during overcharge, but the battery discharge capacitywas 502 mAh, which turned out to be extremely low. Accordingly, it canbe said that desirable C/A value is 0.2 mAh/cm² or more.

INDUSTRIAL APPLICABILITY

The present invention can provide a battery showing a high level ofsafety against overcharge in a wide current range by the interaction ofthe additive to be added to the non-aqueous electrolyte and theelectrode plate having a positive temperature coefficient of resistance.With the use of the non-aqueous electrolyte secondary battery like this,devices having a high level of safety such as cell phone, portableinformation terminal, camcorder, personal computer, PDA, portable audiodevice, electric car, electric source for load leveling and the like canbe provided.

1. A non-aqueous electrolyte secondary battery comprising: a positiveelectrode comprising a positive electrode current collector and apositive electrode material mixture layer formed thereon; a negativeelectrode comprising a negative electrode current collector and anegative electrode material mixture layer formed thereon; and anon-aqueous electrolyte, characterized in that (1) at least one of saidpositive and negative electrodes has a positive temperature coefficientof resistance, (2) said non-aqueous electrolyte contains an additivewhich is stable at the normal operating voltage range of said batteryand is able to polymerize at the voltage exceeding the maximum value ofsaid operating voltage range, wherein said additive is at least oneselected from the group consisting of biphenyl, 3-chlorothiophene,furan, o-terphenyl, m-terphenyl, p-terphenyl, diphenyl ether,2,3-benzofuran, bis(p-tolyl)ether, diallyl ether, allyl butyl ether,3-phenoxy toluene and cyclohexyl benzene, (3) a resistivity at 120° C.of at least one of said positive and negative electrodes is 10⁷ Ω·cm ormore, and (4) a resistance layer having a positive temperaturecoefficient of resistance is formed on a surface of at least one of saidpositive electrode current collector and said negative electrode currentcollector.
 2. The non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein a resistance value at 110 to 130° C. ofat least one of said positive and negative electrodes is 100 times orgreater than a resistance value at 25° C. of the same electrode.
 3. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said positive electrode current collector comprises aluminum andsaid resistance layer comprises a mixture of a conductive particulatematerial and a binder polymer.
 4. The non-aqueous electrolyte secondarybattery in accordance with claim 3, wherein said particulate materialcomprises a carbon material.
 5. The non-aqueous electrolyte secondarybattery in accordance with claim 3, wherein said binder polymer is atleast one selected from the group consisting of polyethylene,ethylene-vinyl acetate copolymer, ethylene-propylene copolymer,ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile and aromatic hydrocarbon having vinyl group-conjugateddiene copolymer.
 6. The non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein said negative electrode currentcollector comprises copper or nickel and said resistance layer comprisesa mixture of a conductive particulate material and a binder polymer. 7.The non-aqueous electrolyte secondary battery in accordance with claim6, wherein said particulate material comprises copper or nickel.
 8. Thenon-aqueous electrolyte secondary battery in accordance with claim 6,wherein said binder polymer is at least one selected from the groupconsisting of polyethylene, ethylene-vinyl acetate copolymer,ethylene-propylene copolymer, ethylene-propylene-vinyl acetatecopolymer, polypropylene, polyacrylonitrile and aromatic hydrocarbonhaving a vinyl group-conjugated diene copolymer.
 9. The non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein aratio of battery capacity C (mAh) to facing area A (cm²) of saidpositive and negative electrodes: C/A value is 0.2 to 6.0 mAh/cm². 10.The non-aqueous electrolyte secondary battery in accordance with claim1, wherein a ratio of battery capacity C (mAh) to facing area A (cm²) ofsaid positive and negative electrodes: C/A value is 0.2 to 4.5 mAh/cm².11. A non-aqueous electrolyte secondary battery comprising: a positiveelectrode comprising a positive electrode current collector and apositive electrode material mixture layer formed thereon; a negativeelectrode comprising a negative electrode current collector and anegative electrode material mixture layer formed thereon; and anon-aqueous electrolyte, characterized in that (1) said positiveelectrode has a positive temperature coefficient of resistance, (2) saidnon-aqueous electrolyte contains an additive which is stable at thenormal operating voltage range of said battery and is able to polymerizeat the voltage exceeding the maximum value of said operating voltagerange, wherein said additive is at least one selected from the groupconsisting of biphenyl, 3-chlorothiophene, furan, o-terphenyl,m-terphenyl, p-terphenyl, diphenyl ether, 2,3-benzofuran,bis(p-tolyl)ether, diallyl ether, allyl butyl ether, 3-phenoxy tolueneand cyclohexyl benzene, (3) a resistance layer having a positivetemperature coefficient of resistance is formed on a surface of saidpositive electrode current collector, (4) said positive electrodecurrent collector comprises aluminum and said resistance layer comprisesa mixture of a conductive particulate material and a binder polymer, and(5) a resistivity at 120° C. of said positive electrode is 10⁷ Ω·cm ormore.
 12. The non-aqueous electrolyte secondary battery in accordancewith claim 11, wherein a resistance value at 110 to 130° C. of saidpositive electrode is 100 times or greater than a resistance value at25° C. of the same electrode.
 13. The non-aqueous electrolyte secondarybattery in accordance with claim 11 wherein said particulate materialcomprises a carbon material.
 14. The non-aqueous electrolyte secondarybattery in accordance with claim 11 wherein said binder polymer is atleast one selected from the group consisting of polyethylene,ethylene-vinyl acetate copolymer, ethylene-propylene copolymer,ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile and aromatic hydrocarbon having vinyl group-conjugateddiene copolymer.
 15. The non-aqueous electrolyte secondary battery inaccordance with claim 11 wherein a ratio of battery capacity C (mAh) tofacing area A (cm²) of said positive and negative electrodes: C/A valueis 0.2 to 6.0 mAh/cm².
 16. The non-aqueous electrolyte secondary batteryin accordance with claim 11 wherein a ratio of battery capacity C (mAh)to facing area A (cm²) of said positive and negative electrodes: C/Avalue is 0.2 to 4.5 mAh/cm².
 17. A non-aqueous electrolyte secondarybattery comprising: a positive electrode comprising a positive electrodecurrent collector and a positive electrode material mixture layer formedthereon; a negative electrode comprising a negative electrode currentcollector and a negative electrode material mixture layer formedthereon; and a non-aqueous electrolyte, characterized in that (1) saidnegative electrode has a positive temperature coefficient of resistance,(2) said non-aqueous electrolyte contains an additive which is stable atthe normal operating voltage range of said battery and is able topolymerize at the voltage exceeding the maximum value of said operatingvoltage range, wherein said additive is at least one selected from thegroup consisting of biphenyl, 3-chlorothiophene, furan, o-terphenyl,m-terphenyl, p-terphenyl, diphenyl ether, 2,3-benzofuran,bis(p-tolyl)ether, diallyl ether, allyl butyl ether, 3-phenoxy tolueneand cyclohexyl benzene, (3) a resistance layer having a positivetemperature coefficient of resistance is formed on a surface of saidnegative electrode current collector, (4) said negative electrodecurrent collector comprises copper or nickel and said resistance layercomprises a mixture of a conductive particulate material and a binderpolymers, and (5) a resistivity at 120° C. of said negative electrode is10⁷ Ω·cm or more.
 18. The non-aqueous electrolyte secondary battery inaccordance with claim 17 wherein a resistance value at 110 to 130° C. ofsaid negative electrode is 100 times or greater than a resistance valueat 25° C. of the same electrode.
 19. The non-aqueous electrolytesecondary battery in accordance with claim 17 wherein said particulatematerial comprises copper or nickel.
 20. The non-aqueous electrolytesecondary battery in accordance with claim 17 wherein said binderpolymer is at least one selected from the group consisting ofpolyethylene, ethylene-vinyl acetate copolymer, ethylene-propylenecopolymer, ethylene-propylene-vinyl acetate copolymer, polypropylene,polyacrylonitrile and aromatic hydrocarbon having a vinylgroup-conjugated diene copolymer.
 21. The non-aqueous electrolytesecondary battery in accordance with claim 17 wherein a ratio of batterycapacity C (mAh) to facing area A (cm²) of said positive and negativeelectrodes: C/A value is 0.2 to 6.0 mAh/cm².
 22. The non-aqueouselectrolyte secondary battery in accordance with claim 17 wherein aratio of battery capacity C (mAh) to facing area A (cm²) of saidpositive and negative electrodes: C/A value is 0.2 to 4.5 mAh/cm².